Line in the Sand—or the Maginot Line?

The Standard & Poor's 500 hovers near a critical level below which the bears will have free rein.

Stock-market bulls are anxiously watching what may be a line in the sand on the charts. Or it could be its Maginot Line.

With short-term trends having turned lower across a broad front, bulls need to see the market mount a rally in short order. If it doesn't, the last few index holdouts will join the Nasdaq with breakdowns below their key,...